1. Field of the Invention
The present invention relates to a steering system for a motor vehicle, which is capable of variably setting a relationship between the steering of steerable vehicle wheels and the operation of an operation mechanism such as a steering wheel.
2. Description of Related Art
A steering system for a motor vehicle has been proposed in which a steering wheel is mechanically disengaged from a steering mechanism for steering vehicle wheels (see, for example, U.S. Pat. No. 5,908,457). This system is adapted to detect the direction and amount of the operation of the steering wheel and apply a driving force from an actuator such as an electric motor to the steering mechanism on the basis of the results of the detection.
This arrangement obviates the need for mechanical coupling between the steering mechanism and the steering wheel, so that upward thrust of the steering wheel can be prevented at collision of the motor vehicle. Further, this arrangement allows for structural simplification and weight reduction of the steering mechanism, and more flexible placement of the steering wheel. Moreover, this arrangement makes it possible to employ other operation mechanisms such as a steering lever and a steering pedal instead of the steering wheel.
In such a motor vehicle steering system, a relationship between the operation of the steering wheel and the operation of the steering mechanism can flexibly be changed through an electrical control. Thus, the maneuverability of the motor vehicle is expected to be drastically improved.
In the case of an ordinary motor vehicle in which a steering wheel and a steering mechanism are mechanically coupled to each other, for example, the dynamic characteristics of the motor vehicle responsive to the steering (over-steering, under-steering, neutral-steering and the like) are virtually determined by various factors (weight, wheel base, gravity center and the like) of the motor vehicle. The motor vehicle is generally designed so as to be driven in a slightly under-steered state.
However, the dynamic characteristics of the motor vehicle vary depending upon the number of passengers and a load on the motor vehicle. As the total weight of the motor vehicle increases, the tendency toward the under-steering generally increases to reduce the steering response of the motor vehicle. That is, the motor vehicle is less easy to corner.
Further, the dynamic characteristics vary from one motor vehicle to another. Therefore, a driver is often confused when driving a motor vehicle other than his familiar motor vehicle, and it takes some time for him to become accustomed to the driving of the motor vehicle.